[BEAM-7911] Fix flakiness in avroio_test.py

[udim]

• Comparison data[last_char_index] == b'B' was never true in Py3 (LHS
  is int, RHS is bytes).
• Fix assertion on exception text.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @username).
• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[udim]

R: @aaltay

[aaltay]

This LGTM.

If the condition was never true, why the test was flaky? Should not it always fail?

[udim]

This LGTM.

If the condition was never true, why the test was flaky? Should not it always fail?

It was intentionally corrupting data by replacing the last byte. It was failing to corrupt the data in 1 out of 256 cases, where the last byte was already 'B'.

[aaltay]

This LGTM.
If the condition was never true, why the test was flaky? Should not it always fail?

It was intentionally corrupting data by replacing the last byte. It was failing to corrupt the data in 1 out of 256 cases, where the last byte was already 'B'.

Ack. Thank you for the fix. Much cleaner.